The progress of lasers in recent years has been remarkable and high output and compact devices of solid laser or semiconductor laser having a light emission region in the range of from near infrared to infrared become easily available. The heat-sensitive lithographic printing plate precursor for direct plate-making, which is used such an infrared laser as a light source, is useful because it can be handled in a bright room and is very preferable in the plate-making operation.
As the heat-sensitive lithographic printing plate precursor for direct plate-making, a positive-working heat-sensitive lithographic printing plate precursor having a heat-sensitive layer (an image-forming layer) comprising a binder resin soluble in an aqueous alkali solution and an infrared absorbing dye or the like, which absorbs light to generate heat, is described, for example, in Patent Document 1 (WO 97/39894) and Patent Document 2 (JP-A-11-44956 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). In the unexposed area of the image-forming layer, the infrared absorbing dye or the like interacts with the binder resin to substantially decreases a solubility of the binder resin. Specifically, the infrared absorbing dye or the like functions as a dissolution inhibiting agent. On the other hand, in the exposed area, the interaction between the infrared absorbing dye or the like and the binder resin is weakened upon the heat generated by the exposure so that the binder resin becomes soluble in an alkali developing solution. Development is conducted using such difference of the solubility between the exposed area and the unexposed area, whereby a lithographic printing plate is prepared.
However, in such a positive-working heat-sensitive lithographic printing plate precursor, there is a problem in that the difference between the dissolution resistance of the unexposed area (image area) to the developing solution and the solubility of the exposed area (non-image area) in the developing solution in various using conditions is yet insufficient, and an excessive development (phenomenon of reduction of film thickness, in which the image layer becomes thin due to the dissolution of the image area) or an inferior development (phenomenon of the occurrence of residual film, in which the non-image area is not completely dissolved and remains) is liable to occur by the fluctuation of the using conditions.
Further, since strength of the image-forming layer is low, there is a problem in that the surface state readily fluctuates, for example, fine scratches are generated even by touching with the surface at the handling, and such slight fluctuation of the surface state, for example, the fine scratches, causes increase of the solubility in the vicinity thereof, as a result, the scratch trace is left in the image area after development, resulting in the occurrence of deterioration of printing durability or inking failure.
Moreover, since the image-forming faculty of heat-developable lithographic printing plate precursor depends upon the heat generation by an infrared laser exposure, there is also a problem that in the vicinity of a support where diffusion of the heat to the support occur, the quantity of heat used for dissolving the image-forming layer is decreased to reduce the difference of solubility between the exposed area and the unexposed area, whereby the reproducibility of highlight becomes insufficient.
Those problems are derived from essential difference in plate-making mechanism between a positive-working heat-sensitive lithographic printing plate precursor and a conventional positive-working photosensitive lithographic printing plate precursor (positive PS plate) for plate-making through UV exposure. Specifically, in the positive PS plate, an image-forming layer contains a binder resin soluble in an aqueous alkali solution and an onium salt or quinonediazide compound. The onium salt or quinonediazide compound performs two functions. One is a function as dissolution inhibiting agent upon interaction with the binder resin in the unexposed area, and the other is a function as dissolution accelerating agent by generating an acid upon decomposition with light in the exposed area.
On the other hand, the infrared absorbing agent or the like contained in the heat-sensitive lithographic printing plate precursor acts only as the dissolution inhibiting agent in the unexposed area, and does not accelerate the dissolution in the exposed area. Therefore, in order to make difference in solubility between the unexposed area and the exposed area in the heat-sensitive lithographic printing plate precursor, it cannot be helped to use as the binder resin a resin essentially having high solubility in an alkali developing solution. Thus, the heat-sensitive lithographic printing plate precursor bears problems in that the reduction of film thickness occurs, in that the scratch resistance is deteriorated and that the states before development are unstable. On the contrary, when the solubility of the binder resin in the alkali developing solution is decreased in order to strengthen the unexposed area, decrease in sensitivity is caused. Accordingly, since the difference in solubility between the unexposed area and the exposed area is small in the heat-sensitive lithographic printing plate precursor, a range of development condition (development latitude) for forming an image by distinguishing of the difference in solubility is narrowly restricted.
Therefore, various investigations have been made to develop a dissolution inhibiting agent having selectivity, which loses the dissolution inhibiting function in the exposed area but maintains the dissolution inhibiting function in the unexposed area. For instance, a technique of adding a light-heat converting agent and a substance that is thermally decomposable and can substantially decrease solubility of the alkali-soluble resin before it is thermally decomposed to a recording layer of positive-working lithographic printing plate precursor for infrared laser is described, for example, in Patent Document 3 (JP-A-7-285275). According to the technique, the dissolution of the recording layer is inhibited and the scratch resistance is improved and on the other hand, in the exposed area the substance thermally decomposable is decomposed by heat generated by the light-heat converting agent to lose the dissolution inhibiting function to the alkali-soluble resin, thereby being capable of increasing the sensitivity.
A heat-sensitive lithographic printing plate precursor comprising a support having thereon a hydrophobic layer containing a polymer soluble in an aqueous alkali solution and an upper layer sensitive to an infrared ray on the hydrophobic layer is described in Patent Document 4 (JP-A-10-250255). The upper layer of the heat-sensitive lithographic printing plate precursor contains carbon black and nitrocellulose or the like. When the heat-sensitive lithographic printing plate precursor is exposed, the upper layer is partially diminished so that the upper layer becomes more permeable to the aqueous alkaline solution. Using the upper layer as a mask, the lower hydrophobic layer is selectively removed upon development to form an image.
Further, it is described in Patent Document 5 (JP-A-2002-251003) that a heat-sensitive lithographic printing plate precursor comprising a lower layer containing a specific water-insoluble and alkali-soluble resin having a sulfonamido group or the like and an upper heat-sensitive layer increasing the solubility in the aqueous alkaline solution upon exposure and containing a water-insoluble and alkali-soluble resin and an infrared absorbing dye exhibits favorable printing durability and development latitude.
However, in such a heat-sensitive lithographic printing plate precursor having an image-forming layer of two-layer type, although the sensitivity can be increased by reducing thickness of the upper heat-sensitive layer, the scratch resistance decreases at the same time. To increase the thickness of upper heat-sensitive layer in order to improve the scratch resistance results in decrease in the sensitivity. Namely, there is a problem of trade-off between the sensitivity and the scratch resistance in the heat-sensitive lithographic printing plate precursor.
Patent Document 1: WO 97/39894
Patent Document 2: JP-A-11-44956
Patent Document 3: JP-A-7-285275
Patent Document 4: JP-A-10-250255
Patent Document 5: JP-A-2002-251003